Crude steel producing methods are roughly classified into a blast-furnace-converter method which produces pig iron from iron ore and refines the pig iron into steel, and an electric furnace method which melts scrap and refines the molten scrap into steel. With the advent of newly industrialized countries including China, the crude steel production quantity has been rapidly increasing on a worldwide basis. Particularly, the supply and demand of iron ore used in the blast-furnace-converter method is tight. Hence, the cost of iron ore is increasing and, at the same time, it is becoming more and more difficult to obtain high-quality iron ore.
In a usual blast furnace operation, zinc which is contained in a charging raw material and is charged into a blast furnace is reduced and evaporated in the furnace, and a part of the zinc is discharged to the outside of the furnace as dust contained in a discharge gas. However, a part of the zinc is oxidized and coagulated as oxide in a part of the furnace where the temperature is low and oxidization potential is high. Zinc oxide is particularly liable to be coagulated on an inner wall of a shaft of the blast furnace thus making coke or ore around the coagulated zinc oxide adhere to each other thus immobilizing a filling material. Such an immobilized portion is referred to as “Ansatz”. “Ansatz” makes the descending of the furnace filling material unstable thus inducing troubles such as “bridging” or “slip”.
Further, in general, zinc-containing dust in a blast-furnace discharge gas which contains a large quantity of iron content is recycled in a sintering process together with other steel-making-plant generated dust. To prevent the occurrence of the above-mentioned drawback attributed to the increase of an amount of zinc charged into the blast furnace, there has been proposed a method for processing dust in which zinc-containing dust is separated into low zinc dust and high zinc dust based on a content of zinc, the low zinc dust is used as a sintering raw material, and the high zinc dust is used as a cement manufacturing raw material (see JP-B-3-62772, for example). Even the process disclosed in JP-B-3-62772 is performed, currently, the amount of zinc charged into a blast furnace from a charging raw material is around an upper limit allowable in a blast furnace operation. The use of iron ore with a high content of zinc as a raw material charged into a blast furnace under such a circumstance increases an amount of zinc charged into the blast furnace. Hence, the use of the iron ore under such a circumstance is difficult.
On the other hand, demand for zinc is rapidly increasing on a worldwide basis in the same manner as iron ore thus giving rise to a drawback that the cost of zinc is increasing. Zinc also has a drawback that a zinc raw material such as sulfide mineral or zinc oxide is in short supply. Zinc is an indispensable metal as a battery raw material, or as a plating material which enhances corrosion resistance of a surface of a steel sheet or the like, for example. Zinc metal is generally produced such that zinc oxide is prepared by oxidizing and roasting sulfide mineral, and zinc oxide is refined by a wet refining method or a dry refining method. Recently, there has been also proposed a method in which crude zinc oxide is prepared by refining iron-making process dust, and crude zinc oxide is used as a zinc refining raw material.
For example, in the case of crude zinc oxide with a zinc concentration exceeding 10 mass %, crude zinc oxide of high concentration can be prepared by applying an intermediate treatment such as a Waelz method or the like, and such crude zinc oxide can be used as a zinc refining raw material. Further, in the case of crude zinc oxide with a zinc concentration exceeding 50 mass %, such crude zinc oxide can be directly used as crude zinc oxide used in zinc refining such as an ISP method or the like, for example.
Under the above-mentioned circumstance where the problem that resources such as the iron raw material and the zinc raw material are in short supply, iron ore which contains zinc larger than a usual content cannot be currently used in a blast furnace so that iron content and zinc content of the iron ore are not effectively utilized.
It could therefore be helpful to provide a method for producing pig iron using iron ore with a high content of zinc which can effectively make use of iron ore with a high content of zinc.
It could therefore be helpful to provide a method for producing pig iron using iron ore with a high content of zinc which can overcome the shortage of supply of both zinc and iron.